Resin-made reflectors for use with light-emitting semiconductor devices, for example, light-emitting diodes (LEDs) are widely known for many years.
In general, reflectors used in light-emitting diodes are made of thermoplastic resins such as PPA (polyphthalamide) with white pigments such as titanium oxide added therein. Practically all light-emitting diodes currently use reflectors molded of these materials.
On the other hand, still higher luminance and durability are required for light-emitting diodes in the field of lighting. A great deal of labor is therefore bestowed on making improvements in brightness per unit power consumption by using shorter-wavelength light-emitting diodes.
Under these circumstances, there is a strong demand from the market for improvements in photoreflectance at low wavelengths and higher durability with respect to reflectors.
As reflectors for light-emitting semiconductor devices, PPA resin has heretofore been used widely. Recently, those molded of thermosetting resins, each of which is composed of an epoxy resin, curing agent and filler and has a reflectance of 80% or higher in the wavelength range of 350 to 800 nm and a thermal conductivity of 1 to 10 W/K, have been exemplified in JP-A 2006-140207 and JP-A 2007-297601.
It is described that a white pigment such as alumina, magnesium oxide, antimony oxide, aluminum hydroxide, barium sulfate, magnesium carbonate or barium carbonate is used to maintain the reflectance at 80% or higher in the above-described wavelength range. To maintain with such a pigment the reflectance at 80% or higher in the wavelength range of 350 to 800 nm, the pigment has to be added in a large amount. Further, the pigments other than alumina involve many problems such as high moisture absorbance and water solubility, and therefore, are not suited for high-reliability reflectors.
Further, the use of PPA resin or epoxy resin in reflectors for light-emitting diodes that emit intense short-wavelength light may cause such inconvenience that the resin is degraded and discolored by the light and the reflectance is lowered.